This invention relates to a process for providing an antireflection film on a substrate, and to the article formed by this process.
It has long been known that it is advantageous to provide various articles, for example lenses, cathode ray tubes, flat panel displays, window films and windshields, with antireflection films which reduce the amount of light reflected from the surface of the article and thus reduce or eliminate xe2x80x9cghostxe2x80x9d images formed by such reflected light. For example, U.S. Pat. Nos. 5,106,671; 5,171,414 and 5,234,748 describe antireflection films which are placed on the inside surface of automobile windshields to reduce the intensity of the image of the instrument panel caused by light reflected from the inside surface of the windshield.
Antireflection coatings on a substrate typically comprise a plurality of inorganic layers, for example a metal or metal oxide layer and a silica layer. (The term xe2x80x9csilicaxe2x80x9d is used herein in accordance with its normal meaning in the antireflection art to mean a material of the formula SiOx where x is not necessarily equal to two. As those skilled in the art are aware, such silica layers are often deposited by chemical vacuum deposition or sputtering of silicon in an oxygen atmosphere, so that the material deposited does not precisely conform to the stoichiometric formula SiO2 of pure silica.) Typically, one surface of a silica layer is exposed, and this exposed surface, which has a high surface energy, as shown by its low contact angle with water, is highly susceptible to fingerprints and other marks. Such marks are extremely difficult to clean, often requiring the use of chemical cleaners.
U.S. Pat. No. 4,765,729 (Taniguchi) describes an anti-reflection optical article, which comprises a substrate bearing a single-layer or multi-layer anti-reflection film having a surface film composed of an inorganic substance, and a coating of an organic substance containing a curing material formed on the surface of the anti-reflection film, wherein the surface reflectance of the optical article is lower than 3% and the stationary contact angle to water is at least 60xc2x0. The inorganic substance is preferably silica and the preferred curing material is a silanol-terminated polysiloxane. According to this patent, the thickness of the organic substance should be in the range of 0.0005 to 0.5 xcexcm (0.5 to 500 xcexcm), especially 0.001 to 0.3 xcexcm (1 to 300 xcexcm). The provision of the layer of organic substance is stated to increase the scratch and stain resistance of the optical article.
However, this patent gives no directions for controlling the thickness of the organic substance within the very broad range which it suggests for such thickness, and all of the worked examples use a dip coating technique which would lead to very thin coatings the thickness of which would be expected to vary significantly over the surface of the coated substrate, since the coating solution will accumulate at the lowest point on the substrate. Furthermore, in these worked examples, a substrate such as a lens is first provided with antireflection layers of metal oxide and silica to a total thickness of xcex/4 and then the organic substance is applied by dip coating. In these circumstances, proper antireflection properties will be obtained only if the organic substance is very thin, less than 10 nm, so that it has essentially no effect on the optical properties of the antireflection coating, and as already indicated, it is difficult to achieve uniformity in such very thin coatings.
Perhaps the most effective antireflection film available commercially is that sold by Southwall Technologies, 1029 Corporation Way, Palo Alto, Calif. 94303. This material comprises a 180 xcexcm poly(ethylene terephthalate) substrate provided with an abrasion-resistant hard coat, and then successively with a 17 nm indium tin oxide (ITO) layer, a 23 nm silica layer, a 95 nm ITO layer, an 84 nm silica layer and finally a thin xe2x80x9clubricationxe2x80x9d layer, which is formed from a fluoropolymer and is stated to improve the scratch resistance and the susceptibility of the surface to marking.
This complex film possesses excellent antireflection characteristics, but is so expensive (approximately US$10 per square foot, US$100 mxe2x88x922) as to preclude its use in many applications where antireflection films are desirable. Much of the high cost of this film can be attributed to the 95 nm ITO layer and 84 nm silica layer; since these layers are typically formed by sputtering, and the cost of a sputtered layer is directly proportional to its thickness. Furthermore, if it is desired to produce large quantities of such a complex film on a production line basis, the need for four separate sputtering stations, all of which must be maintained under high vacuum, results in a complex and costly apparatus.
It has now been found that providing a xe2x80x9cthickxe2x80x9d (i.e., optically active) polymer layer of carefully controlled refractive index above an inorganic antireflection layer or layers, the thickness(es) of the inorganic layer(s) can be greatly reduced, thereby reducing the overall cost of the antireflection coating, especially when the inorganic layer(s) is/are applied by a process such as sputtering or chemical vapor deposition in which the residence time of the substrate within the coating apparatus is directly proportional to the thickness of the required layer. Also, an antireflection coating using such a thick polymer layer, which can readily be applied with good uniformity by solution or other coating techniques, has good scratch and stain resistance.
Accordingly, this invention provides an article having an antireflection film. This article comprises a substrate carrying an inorganic antireflection layer, and in contact with the antireflection film and forming the outer surface of the antireflection film, a polymer layer formed by curing a curable composition in situ on the inorganic antireflection layer, the polymer layer having a refractive index not greater than about 1.53 over the wavelength range of 400 to 700 nm and a thickness of from about 20 to about 200 nm.
This invention also provides a process for providing an antireflection film on a substrate. This process comprises depositing an inorganic antireflection layer on the substrate; depositing a layer of a curable composition on the inorganic antireflection layer; and effecting free radical curing of the deposited curable composition to form a polymer layer having a thickness of from about 20 to about 200 nm and a refractive index not greater than about 1.53 over the wavelength range of 400 to 700 nm.
In the present process, the curing of the curable composition may be effected by cross-linking of one or more polymers or oligomers, or by polymerization of one or more monomers or oligomers, or by a combination of both cross-linking and polymerization. Such curing techniques are familiar to those skilled in polymer technology.